Device For Temperature Measurement
The invention relates to a device for temperature measurement.
Such devices which are known in the art for contactless temperature measurement comprise a detector for receiving heat radiation emanating from a measurement spot on an object of measurement, an optical system for imaging the heat radiation emanating from the measurement spot onto the detector and a sighting arrangement for identifying the position and size of the measurement spot on the object of measurement by means of visible light. A further processing arrangement which converts the detector signal into a temperature indication is also connected to the detector.
In this case the optical system is so designed that at a certain measurement distance for the most part only heat radiation from a certain area of the object of measurement, namely the so-called measurement spot, is focussed onto the detector. In most cases the size of the measurement spot is defined by the area from which 90% of the heat rays focussed onto the detector strike. However, applications are also known in which there are reference to values between 50% and 100%.
The pattern of the dependence of the size of the measurement spot upon the measurement distance depends upon the design of the optical system. A fundamental distinction is made between distant focussing and close focussing. In distant focussing the optical system images the detector into infinity and in close focussing it images it onto the focus plane. In the case of distant focussing it is necessary to deal with a measurement spot which grows linearly with the measurement distance, whereas in close focussing the measurement spot will first of all become smaller with the measurement distance and after the focus plane will enlarge again if the free aperture of the optical system is greater than the measurement spot in the focus plane. If the measurement spot in the focus plane is greater than the free aperture of the optical system, then the measurement spot is also enlarged with the measurement distance even before the focus plane. Only the increase in the size of the measurement spot is smaller before the focus plane than after it.
In the past various attempts were made to render the position and size of the measurement spot, which is invisible per se, visible by illumination. According to JP-A-47-22521 a plurality of rays which originate from several light sources or are obtained by reflection from a light source are directed along the marginal rays of a close-focussed optical system onto the object of measurement. In this way the size and position of the measurement spot for a close-focussed system can be rendered visible by an annular arrangement of illuminated points around the measurement spot. U.S. Pat. No. 5,368,392 describes various methods of outlining measurement spots by laser beams. These include the mechanical deflection of one or several laser beams as well as the splitting of a laser beam by a beam divider or a fibre optic system into several single beams which surround the measurement spot.
A sighting system is also known in the art which uses two laser beams to describe the size of the measurement spot. This system uses two divergent beams emanating from the edge of the optical system to characterise a close-focussed system and two laser beams which intersect in the focus point to characterise a close-focussed optical system.
All known sighting arrangements are either only useful for a certain measurement distance or require relatively complex adjustment and are often quite expensive.